Fluoropolymer is usually olefinic polymer which consists of partially or fully fluorinated olefinic monomers such as vinylidene fluoride (CH2¼CF2) and tetrafluoroethylene (TFE) (CF2¼CF2). These polymers have been covered in great detail in a number of references. More specialty fluorinated polymers include per-fluoroethers, fluoroacrylates, and fluorosilicones which are used in significantly smaller volume than olefinic fluoropolymers.
Commercial fluoropolymers include homopolymers and copolymers. Homopolymers contain 99% or more by weight one monomer and 1% or less by weight of another monomer according to the convention by American Society for Testing Materials (ASTM). Copolymers contain more than 1% or more by weight of one or more comonomers. The major commercial fluoropolymers are based on three monomers:
TFE, vinylidene fluoride (VF2), and to a lesser extent chlorotrifluoroethylene (CTFE). Examples of comonomers include perfluoromethyl vinyl ether (PMVE), perfluoroethyl vinyl ether (PEVE), perfluoropropyl vinyl ether (PPVE), hexafluoropropylene (HFP), CTFE, perfluorobutyl ethylene (PFBE), and exotic monomers such as 2,2-bistri-fluoromethyl-4,5-difluoro-1,3-dioxole.
A good rule of thumb to remember is that increasing the fluorine content of a polymer molecule increases its chemical and solvent resistance, flame resistance, and photostability; improves its electrical properties such as lower dielectric constant; lowers coefficient of friction; raises melting point; increases its thermal stability; and weakens its mechanical properties. Solubility of polymers in solvents usually decreases by increasing the fluorine content of the molecule.
Fluoropolymer Classification
The serendipitous discovery of PTFE in 1938 by Roy Plunkett of DuPont Company began the era of fluoropolymers.PTFE has found thousands of applications because of its unique properties. Various fluoroplastics have been developed since the discovery of PTFE. A number of companies produce these plastics around the world. Fluoropolymers are divided into two classes of perfluorinated and partially fluorinated polymers. Perfluorinated fluoropolymers are homopolymers and copolymers of TFE. Some of the comonomers may contain a small amount of elements other than C or F.
Polymer Development History
PTFE cannot be fabricated by melt-processing techniques because of its high viscosity. Melt-processible fluoropolymers have been developed by copolymerization of TFE. FEP, a copolymer of TFE and HFP, has a lower maximum continuous use temperature than PTFE (200 C vs. 260 C) because of the deterioration of mechanical properties. PFA, a copolymer of TFE with PPVE or PEVE, offers thermal stability, melt-processibility and maximum continuous use temperatured 260 C. Both FEP and PFA are considered perfluoropolymers.
Copolymers of ethylene with tetrafluoroethylene (ETFE) and chlorotrifluoroethylene (ECTFE) are mechanically stronger than perfluoropolymers, accompanied with tradeoffs of reduction in their chemical resistance and continuous use temperature and an increase in the coefficient of friction.
Amorphous copolymers of TFE are soluble in special halogenated solvents and can be applied to surfaces as a polymer solution to form thin coatings. The dried coating is as resistant to almost as many chemicals as PTFE.
Post time: Jul-22-2017